Burning rate accelerating method

ABSTRACT

1. The method of accelerating the burning rate of a solid propellant grain, said grain consisting essentially of a cured, intimate mixture of a propellant mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of: A. INCORPORATING INTO SAID MIX AND UNIFORMLY DISTRIBUTING THERETHROUGH FINE, ELONGATED, FERROMAGNETIC METAL PARTICLES, THE AMOUNT OF SAID PARTICLES BEING FROM ABOUT 0.1 PERCENT BY WEIGHT TO ABOUT 5.0 PERCENT BY WEIGHT OF MIX EXCLUDING THE WEIGHT OF SAID METAL PARTICLES. B. Subjecting the mix containing said metal particles to a magnetic field of sufficient strength to at least partially orient the lengthwise axis of some said metal particles substantially perpendicular to the combustion plane of said grain before said mix cures whereby upon curing of the mix said metal particles are rigidly held in the oriented position.

[73] Assignee United States Patent [72] Inventor John M. Lenoir Thousand Oaks, Calif. [21] Appl. No. 334,664

Dec. 30, 1963 Nov. 2, 1971 The United States of America as represented by the Secretary of the Army [22] Filed [45] Patented [54] BURNING RATE ACCELERATING METHOD 8 Claims, 3 Drawing Figs.

[52] U.S. Cl 264/3 C, 102/102, 86/20, 149/19 [51] Int. Cl. C06b 21/02 [50] Field of Search 102/98, 38, 39; 86/1; 60/35.6; 264/34 3,049,043 Milani BINDER OXIDANT FERROMAGNETIC PARTICLES PROPELLANT MIXING APPLYING MAGNETIC FIELD CURING ORIENTED MIXTURE Primary Examiner Robert F. Stahl Attorneys-Harry M. Saragovitz, Edward J. Kelly and Herbert Berl CLAIM: 1. The method of accelerating the burning rate of a solid propellant grain, said grain consisting essentially of a cured, intimate mixture of a propellant mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of:

a. incorporating into said mix and uniformly distributing therethrough fine, elongated, ferromagnetic metal particles. the amount of said particles being from about 0.1 percent by weight to about 5.0 percent by weight of mix excluding the weight of said metal particles.

b. Subjecting the mix containing said metal particles to a magnetic field of sufficient strength to at least partially orient the lengthwise axis of some said metal particles substantially perpendicular to the combustion plane of said grain before said mix cures whereby upon curing of the mix said metal particles are rigidly held in the oriented position.

LOADING MIXTURE IN CONTAINER POSITIONING MAGNETIC MANDREL I I I APPLYING MAGNETIC FIELD TO ORIENT THE PARTICLES PERPENDICULAR TO MANDREL AXIS CURING ORIENTED MIXTURE PATENTED 2 3 6 1 7, 586

SHEET 1 0F 2 I 300 WITH ORIENTED STRANDS 3 w LI 5 (I) h) {I D.

UNORIENTED STRANDS NO STRANDS o 2 0 2 4 s 8 IO l2 l4 l6 l8 TIME, seconds FIG. l

WITH ORIENTED STRANDS 5 750 a NO STRANDS l Y o 2 4 6 8 IO l2 TIME, seconds FIG. 2 John M. Lenoir,

PATENT EuIIIIv 2 Ian 3,617, 585

SHEET 2 BF 2 BINDER OXIDANT FERROMAGNETIC PARTICLES LOADING PROPELLANT MIXTURE IN MIXING CONTAINER APPLYING POSITIONING MAGNETIC MAGNETIC FIELD MANDREL I I I I V APPLYING MAGNETIC g wg FIELD TO ORIENT THE MIXTURE PARTICLEs PERPENDICuLAR To MANDREL AxIs I I I V CuRING ORIENTED MIxTuRE FIG. 3

John M. Lenoir,

INVENTOR.

)WM W M W15 BURNING RATE ACCELERATING METHOD This invention relates to a method for accelerating or increasing the burning rate of solid propellant grains and particularly to a method for accelerating the burning rate of solid rock propellants.

The basic solid propellant burning rate equation is:

where r is the burning rate in inches per second, P, is the combustion chamber pressure in pounds per square inch, k is a constant which varies with the ambient grain temperature, and n is a constant known as the burning rate exponent. Various means have been devised to increase the propellant burning rate such as the use of burning rate catalysts as taught by U.S. Pat. No. 2,963,356. Increasing the burning rate of a propellant increases the amount of propellant consumed per unit of time and thus high burning rate permit the achievement of high thrust in shorter periods of time.

From the above equation, it is apparent that the burning rate of a given solid propellant grain varies with the temperature of the grain at the time of ignition, or the ambient temperature. Thus, a solid propellant grain at a temperature of 1 F. for example would burn at a higher rate than the same grain at a temperature of 40 F. This is due to the fact that solid propellant grains are very poor thermal conductors and the heat produced during combustion of a portion of the grain results in only a negligible increase, if any, in temperature of the remainder of the grain. Therefore, for practical purposes, the temperature of the last portion of the grain to burn is essentially equal to the temperature of the grain at the start of combustion.

According to the present invention, the burning rate of any solid propellant grain is increased by incorporating into the propellant grain ferromagnetic metal particles in such a manner that upon combustion of the grain, part of the heat of combustion is conducted by the metal particles into the region of unburned propellant grain thereby raising the temperature of the grain and thus increasing its burning rate.

In accordance with the foregoing, it is an object of the present invention to provide an improvement in solid propellent grains by increasing the burning rate of these grains.

Another object of the invention is to provide a method ofincreasing the burning rate of solid propellant grains by orienting elongated ferromagnetic metal particles perpendicular to the combustion plane of the grains.

A further object of the instant invention is to provide a method for accelerating the burning rate of solid propellant grains by orienting elongated ferromagnetic metal particles perpendicular to the burning plane of the grain with a magnetic field.

A still further object of the invention is to provide a method for accelerating the burning rate of internally perforated solid propellant grains by orienting elongated ferromagnetic metal particles perpendicular to the burning plane of the grain with the magnetic field from a magnetic mandrel.

The manner in which these and other objects can be accomplished will become apparent from the following detailed description wherein:

H65. 1 and 2 are pressure-time curves comparing the burning rates of solid propellant grains incorporating the present invention with the burning rate of the same type grains minus the oriented metal particles.

FIG. 3 is a schematic flowsheet showing the steps employed in preparing propellant grains according to the methods of the present invention. The methods and improvements of the present invention are applicable to the acceleration of any solid propellant grain, particularly those consisting of a cured, intimate mixture of a liquid polymeric binder, fuel particles, and an oxidant therefor, since the invention is independent of the particular composition of the grain. The oriented elongated ferromagnetic metal particles conduct heat ahead of the burning plane thereby increasing the temperature of the grain and, as shown by the burning rate equation, raising the temperature of the propellant grain increases the burning rate.

The fuel particles mentioned can be high-energy fuel additives such as boron, aluminum, magnesium, magnesium-aluminum alloys and the like or the fuel particles can be the polymeric binder itself or both of these. The oxidizer can be any of the conventional oxidizers as exemplified by the alkali and alkaline perchlorates, chlorates, and nitrates, ammonium nitrate, and ammonium perchlorate. Suitable polymers include the polysulfides, polyurethanes, polydienes, polydieneacrylic acid copolymers and the like. Double-base propellant compositions also exhibit burning rate increases when incorporating the present invention. Examples of solid propellent formulations whose burning rates can be increased according to the present invention and methods for making these formulations are shown in the following U.S. Pat. Nos. 2,967,097; 2,970,898; 2,988,436; 2,992,908; 2,992,911; 2,997,375; 3,003,310; 3,003,861; 3,005,692; 3,036,939; 3,044,9l 1. However, it will be obvious to those skilled in the art that there are many other solid propellent compositions which can be used with the instant invention.

According to the present invention, fine, elongated ferromagnetic metal particles are added to the solid propellant mix at sometime prior to curing the mix. Thus the ferromagnetic particles can be added when the rest of the propellant components are being mixed together or they can be added after all other ingredients have been mixed. The only critical aspect of adding the metal particles to the mix is that they must be added before the mix cures and, after adding them, the mix should be thoroughly agitated to ensure substantially uniform distribution of the metal particles throughout the mix. In other words the ferromagnetic metal particles are incorporated into the mix in the same manner as any of the conventional metal additives such as aluminum, boron, magnesium. and the like. Before the mix cures, a magnetic flux is applied to the mix to orient the particles perpendicular to the intended burning plane of the cured grain.

As used herein, the terminology fine, elongated ferromagnetic metal particles" is intended to encompass metal particles or iron, nickel, or cobalt or alloys of these metals with each other or with other metals provided that the particles are suffciently responsive to a magnetic field to orient themselves with respect to flux lines of a magnetic field in the same manner as iron filings. The term fine" is intended to describe particles of an average thickness of about 0.02 inch or less while elongated means that the length of the particle is at least greater than the average thickness; at least two or three times the thickness when the average thickness is about 0.0l inch or larger and preferably l0 or more times the thickness. In the case of particles of small average thickness, for example those having an average thickness of 0.001 inch to 0.009, the length can be up to about 700 times the thickness as long as the length of the particle does not exceed about 0.75 inch. The thing to be considered with respect to using the invention is that the metal particles have the primary function of conducting heat through the propellant grain to raise the temperature of the grain before it burns. It is obvious, therefore, that a larger number of particles of a smaller thickness (0.00l inch to 0.009 inch) and a length of about 0.25 inch to about 0.5 inch will more uniformly conduct heat through the grain than a fewer number of larger particles representing the same weight but having a larger average thickness and less length. For this reason, the use of larger numbers of particles having smaller average thickness and greater lengths are preferred.

The amount of metal particles to be used depends on the density of the particular ferromagnetic metal particles to be used and the desired increase in burning rate. Thus, a few tests with a given type of propellant grain and the metal particles will enable those skilled in the art to determine the particular amounts required for their specific application of the invention. Generally, however, the amount of metal particles will vary from about 0.l percent by weight to about 5 percent by weight of the solid propellant grain based on the weight of the grain without the'ferromagnetic particles. Thus in manufacturing a grain to weigh pounds without any ferromagnetic particles, about 0.l pounds to about 5 pounds of metal particles can be added to the propellant mix. Using soft iron particles, excellent burning rate increases have been achieved with 1 percent by weight of the preferred smaller particles.

A readily available and economical source of ferromagnetic particles is commercial soft iron wire. The wire can be cut into whatever lengths desired and easily obtained in a wide range of diameters.

After the particles are fairly uniformly distributed in the uncured mix and the mix has been poured into a mold or other container such as a rocket motor casing, a magnetic flux is applied to the uncured mix so that the ferromagnetic particles in the mix can orient themselves in response to the magnetic flux before the mix cures or solidifies. The flux is applied for a period of time with sufficient field strength to orient at least some of the particles and, preferably, substantially all of the particles if the highest burning rate is desired. While the flux is being applied, the curing action of the mix causes the mix to thicken so that as the particles orient themselves, they are supported in the oriented position by the thickening mix. Upon curing or solidifying, the oriented particles are held rigidly in place.

The amount of time required to orient the strands depends on the strength of the magnetic field, the responsiveness of the metal particles, the thickness of the uncured mix, and the like. Therefore, a few trails with a given flux, the selected ferromagnetic particles, and a specific type mix will permit the determination of conditions for any specific application of the invention. The mix containing the particles should not be vibrated to any appreciable extent during application of the magnetic flux or curing since the ferromagnetic particles tend to settle under this condition.

The magnetic field should be such that when the particles orient themselves in response to the flux lines, they should be substantially perpendicular to the intended burning plane of the grain. Thus, the particles in an end burning cylindrical grain should be parallel to the center axis of the grain while the particles in a perforated cylindrical grain which is designed to burn outward from the center should be perpendicular to the center axis of the perforation To achieve the parallel orientation in the end burning grain, the north pole of one magnet can be placed near or at one end of the grain and the south pole of another magnet placed at the other end of the grain. To orient the particles perpendicular to the center axis of an internal perforation in the grain, all that is required is that the mandrel used to form the perforation be one end ofa magnet (that is, the region around one pole of the magnet). This of course requires that the other end of the mandrel project from the end of the grain but this is no problem since it facilitates removal of the grain. In fact, to best achieve substantially perpendicular orientation of the particles around the mandrel, the mandrel should consist of not more than half the length of the magnet and preferably about one-third to onefourth of the length. Even smaller fractional lengths are advantageous but economic considerations generally outweigh any advantage when less than one-fourth of the length of the magnet is used as the mandrel. Moreover, those skilled in the art will be aware of many other means for applying a magnetic flux to the grain.

The type of magnet used, that is electromagnet or permanent magnet, is immaterial to the practice of the invention. Availability, cost, size, and other considerations will determine the choice of magnet. If soft steel is used as the mandrel, the mandrel can be attached to an electromagnet and when the electromagnet is energized, the mandrel becomes part of the magnet and orients the particles.

A magnet is used as a mandrel in the same manner as any other mandrel material. Grease or some plastic coating, preferably Teflon (trademark for tetrafluoroethylene resins) are used to coat the mandrel to prevent the propellent from adhering to the mandrel surface and thus hindering its removal.

The actual increase in burning rate achieved through the present invention is demonstrated with a conventional polysulfide polymer, JPL-l26 propellant, having the following compositions:

' Liquid polysulfide polymer available from Thiokol Chemical Corporation.

These ingredients were stirred together to form a propellant mix and then cast in cylindrical molds according to the procedure for preparing polysulfide propellant grains as shown in US. Pat. No. 2,977,376. The only difference being that iron particles (SAE I030 mild steel wire) about 0.375 inch in length and 0.006 inch in diameter were incorporated into some of the mixes in the amount of 1 percent by weight based on the total weight of the other ingredients.

The propellant grains were cast in aluminum molds 4.5 inches in diameter by 6 inches in length with a l.75-inch diameter Teflon coated soft steel mandrel centered in the molds. An electromagnet was attached to the soft steel mandrel (whereby the mandrel thus became magnetized) and a strong magnetic flux was applied to the mandrel. Upon curing. the grains were ignited and burned to completion in test motors. The results are shown in FIG. 2, a burning rate increase of 22 percent over the identical grain without the wire strands.

Following the same procedures and using the same polysulfide formulations given above, additional grains were prepared (1) without the wire strands, (2) with 1 percent by weight of wire strands added to the formulation without the use ofa magnetic mandrel to orient the strands, and (3) with 1 percent by weight of oriented strands. In this case, the magnetic flux was applied to the mandrel until the mix had cured to the extent that it had gelled. The results achieved upon burning these grains in test motors are shown in FIG. I wherein the oriented wire strands cause about one-third increase in pressure in about three-fourths the time required by the grains without wire strands and grains with unoriented wire strands.

The results shown in FIG. I and FIG. 2 illustrate the appreciable increase in propellant grain burning rates achievable by the invention. Results with other solid propellant grains using different polymeric binders are essentially the same.

It is particularly noteworthy that the pressure-time curves for the oriented wire strands in FIG. 1 and FIG. 2 are free of any flattening. This is desirable since it established that there is no substantial concentration of the strands near the mandrel by the magnetic flux.

This invention can also be applied to accelerating the burning rate of solid propellant grains having little or no oxidant in the grain such as the solid grains used in hydbrid-fueled rocket engines wherein a fluid oxidizer is used in the combustion of a solid oxidizer. The application of the invention to such grains is identical to those containing an oxidant.

The above-detailed discussion is for the purpose of illustrating the invention and no undue limitation should be attributed to the scope of the invention as a result thereof except as reflected in the appended claims.

I claim:

I. The method of accelerating the burning rate of a solid propellant grain, said grain consisting essentially of a cured, intimate mixture of a propellant mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of:

a. incorporating into said mix and uniformly distributing therethrough fine, elongated, ferromagnetic metal particles, the amount of said particles being from about 0.1 percent by weight to about 5.0 percent by weight of mix excluding the weight of said metal particles.

b. subjecting the mix containing said metal particles to a magnetic field of sufficient strength to at least partially orient the lengthwise axis of some said metal particles substantially perpendicular to the combustion plane of said grain before said mix cures whereby upon curing of the mix said metal particles are rigidly held in the oriented position.

2. The method according to claim 1, wherein said metal particles are characterized by an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch.

3. The method according to claim 1 wherein said ferromagnetic metal particles are selected from the group consisting of iron and iron alloy particles.

4. The method of accelerating the burning rate of an internally perforated solid propellant grain, said grain being a cured, intimate mixture ofa propellant mix, said mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of:

a. incorporating into said mix and uniformly distributing therethrough fine, elongated ferromagnetic metal particles, the amount of said particles being from about 0.1 percent by weight to about 5.0 percent by weight of said mix excluding the weight of said metal particles,

b. placing the uncured mix containing said metal particles into a suitable container, and

c. positioning in said container a magnetic mandrel having sufficient magnetic field strength to at least partially orient some of said metal particles substantially perpendicular to the center axis of said mandrel before said mix cures whereby upon curing the mix said metal particles are rigidly held in the position they achieved as a result of the magnetic field.

5. The method according to claim 4 wherein said magnetic mandrel consists of less than half of the total length of the magnet forming the mandrel.

6. The method according to claim 4 wherein said metal particles are characterized by an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch.

7. The method according to claim 4 wherein said ferromagnetic metal particles are selected from the group consisting of iron and iron alloy particles.

8. The method of accelerating the burning rate of an extremely perforated solid propellant grain, said grain being a cured intimate mixture of a propellant mix, said mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of:

a. incorporating into said mix and uniformly distributing therethrough fine, elongated ferromagnetic particles having an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch, the amount of said particles being from about 0.1

, percent by weight to about 5.0 percent by weight of said mix, excluding the weight of said metal particles,

b. placing the uncured mix containing said metal particles into a suitable container, and

c. positioning in said container a magnetic mandrel conforming to the shape of the desired internal perforation, said mandrel having sufficient magnetic field strength to at least partially orient some of said metal particles substantially perpendicular to the center axis of said mandrel before said mix cures, said mandrel being less than half of the total length of the magnet forming the mandrel, whereby upon curing said metal particles are rigidly held in the position they achieved as a result of the magnetic field. 

1. THE METHOD OF ACCELERATING THE BURNING RATE OF A SOLID PROPELLANT GRAIN, SAID GRAIN CONSISTING ESSENTIALLY OF A CURED, INTIMATE MIXTURE OF A PROPELLANT MIX CONSISTING ESSENTIALLY OF AN UNCURED, LIQUID POLYMERIC BINDER, FUEL PARTICLES, AND AN OXIDANT THEREFOR, SAID METHOD COMPRISING THE STEPS OF: A. INCORPORATING INTO SAID MIX AND UNIFORMLY DISTRIBUTING THERETHROUGH FINE, ELONGATED, FERROMAGNETIC METAL PARTICLES, THE AMOUNT OF SAID PARTICLES BEING FROM ABOUT 0.1 PERCENT BY WEIGHT TO ABOUT 5.0 PERCENT BY WEIGHT OF MIX EXCLUDING THE WEIGHT OF SAID METAL PARTICLES. B. SUBJECTING THE MIX CONTAINING SAID METAL PARTICLES TO A MAGNETIC FIELD OF SUFFICIENT STRENGTH TO AT LEAST PARTIALLY ORIENT THE LENGTHWISE AXIS OF SOME SAID METAL PARTICLES SUBSTANTIALLY PERPENDICULAR TO THE COMBUSTION PLANE OF SAID GRAIN BEFORE SAID MIX CURES WHEREBY UPON CURING OF THE MIX SAID METAL PARTICLES ARE RIGIDLY HELD IN THE ORIENTED POSITION.
 2. The method according to claim 1, wherein said metal particles are characterized by an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch.
 3. The method according to claim 1 wherein said ferromagnetic metal particles are selected from the group consisting of iron and iron alloy particles.
 4. The method of accelerating the burning rate of an internally perforated solid propellant grain, said grain being a cured, intimate mixture of a propellant mix, said mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of: a. incorporating into said mix and uniformly distributing therethrough fine, elongated ferromagnetic metal particles, the amount of said particles being from about 0.1 percent by weight to about 5.0 percent by weight of said mix excluding the weight of said metal particles, b. placing the uncured mix containing said metal particles inTo a suitable container, and c. positioning in said container a magnetic mandrel having sufficient magnetic field strength to at least partially orient some of said metal particles substantially perpendicular to the center axis of said mandrel before said mix cures whereby upon curing the mix said metal particles are rigidly held in the position they achieved as a result of the magnetic field.
 5. The method according to claim 4 wherein said magnetic mandrel consists of less than half of the total length of the magnet forming the mandrel.
 6. The method according to claim 4 wherein said metal particles are characterized by an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch.
 7. The method according to claim 4 wherein said ferromagnetic metal particles are selected from the group consisting of iron and iron alloy particles.
 8. The method of accelerating the burning rate of an extremely perforated solid propellant grain, said grain being a cured intimate mixture of a propellant mix, said mix consisting essentially of an uncured, liquid polymeric binder, fuel particles, and an oxidant therefor, said method comprising the steps of: a. incorporating into said mix and uniformly distributing therethrough fine, elongated ferromagnetic particles having an average thickness of about 0.001 inch to about 0.02 inch and a length of about 0.1 inch to about 0.75 inch, the amount of said particles being from about 0.1 percent by weight to about 5.0 percent by weight of said mix, excluding the weight of said metal particles, b. placing the uncured mix containing said metal particles into a suitable container, and c. positioning in said container a magnetic mandrel conforming to the shape of the desired internal perforation, said mandrel having sufficient magnetic field strength to at least partially orient some of said metal particles substantially perpendicular to the center axis of said mandrel before said mix cures, said mandrel being less than half of the total length of the magnet forming the mandrel, whereby upon curing said metal particles are rigidly held in the position they achieved as a result of the magnetic field. 